The present invention relates generally to magnetron sputtering apparatus, and more particularly to rotating magnetrons.
DC reactive sputtering is the deposition process most often used for large area commercial coating applications, such as the application of thermal control coatings to architectural and automobile glazings. In this process, the articles to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. This system may be referred to as a continuous in-line system or a glass coater.
Inside the vacuum chambers, a sputtering gas discharge is maintained at a partial vacuum at a pressure of about 3 millitorr. The sputtering gas comprises a mixture of an inert gas, such as argon, with a small proportion of a reactive gas, such as oxygen, for the formation of oxides.
Each chamber contains one or more cathodes held at a negative potential of about -200 to -1000 volts. The cathodes may be in the form of elongated rectangles, the length of which spans the width of the line of chambers. The cathodes are typically 0.10 to 0.30 meters wide and a meter or greater in length. A layer of material to be sputtered is applied to the cathode surface. This surface layer or material is known as the target or the target material. The reactive gas forms the appropriate compound with the target material.
Ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, are deposited on a substrate, such as a glass sheet, passing beneath the target. The atoms react on the substrate with the reactive gas in the sputtering gas discharge to form a thin film.
The architectural glass coating process was made commercially feasible by the development of the magnetically-enhanced planar magnetron. The planar magnetron has an array of magnets arranged in the form of a closed loop and mounted in a fixed position behind the target. A magnetic field in the form of a closed loop is thus formed in front of the target. The field causes electrons from the discharge to be trapped in the field and travel in a spiral pattern, which creates a more intense ionization and higher sputtering rates. Appropriate water cooling is provided to prevent overheating of the target. The planar magnetron is further described in U.S. Pat. No. 4,166,018.
A disadvantage of the planar magnetron is that the target material is only sputtered in the narrow zone defined by the magnetic field. This creates a "racetrack"-shaped sputtering zone on the target. Thus, a "racetrack"-shaped erosion zone is produced as sputtering occurs. This causes a number of problems. For example, (1) localized high temperature build-up eventually limits the power at which the cathodes can operate, and (2) only about 25 percent of the target material is actually used before the target must be replaced.
The rotary or rotating magnetron was developed to overcome some of the problems inherent in the planar magnetron. The rotating magnetron uses a substantially cylindrical cathode and target. The cathode and target are rotated continually over a magnetic array which defines the sputtering zone. As such, a new portion of the target is continually presented to the sputtering zone which eases the cooling problem, allowing higher operating powers. The rotation of the cathode also ensures that the erosion zone comprises the entire circumference of the cathode covered by the sputtering zone. This increases target utilization. The rotating magnetron is described further in U.S. Pat. Nos. 4,356,073 and 4,422,916, the entire disclosures of which are hereby incorporated by reference.
The rotating magnetron requires bearings to permit rotation, and vacuum seals for the drive shaft, the electrical conduits and the cooling conduits. Vacuum and rotary water seals have been used to seal around the drive shaft and the conduits which extend between the coating chamber and the ambient environment. However, such seals have a tendency to develop leaks under conditions of high temperature and high mechanical loading. Various mounting, sealing and driving arrangements for rotating magnetrons are described in U.S. Pat. Nos. 4,443,318; 4,445,997; and 4,466,877, the entire disclosures of which are also hereby incorporated by reference. These patents describe rotating magnetrons mounted horizontally in a coating chamber and supported at both ends.
It is often preferable, however, to support the magnetron at one only end by a cantilever mount. However, the cantilever mounting arrangement produces the highest bearing loads. Several examples of cantilever mounted rotary magnetrons are given in Design Advances and Applications of the Rotatable Magnetron, Proceedings of the 32nd National Symposium of the American Vacuum Society, Vol. 4, No. 3, Part 1, pages 388-392 (1986), the entire text of which is hereby incorporated by reference.
If the bearings and seals are located within the coating chamber, space is used which would otherwise be available for cathode length. Further, if the cathode is supported at each end, more chamber space is used than if the cathode were supported at only one end. In either case, the rotating magnetron's efficiency is reduced since the useful sputtering zone of the cathode may not occupy the full width of the coating chamber. Thus, throughput is decreased as only narrow substrates can be coated.
A cantilever mounted magnetron usually includes a bearing housing containing a drive shaft, a rotary vacuum seal, and at least two bearings spaced along the drive shaft, one of which may function as a shaft seal. This bearing assembly is usually bulky and heavy, because of its load requirements. Also, since the cantilever leverage is best maximized by increasing the separation of the individual bearings in the assembly, the assembly is relatively long compared to the cathode length.
Generally, a major portion of the bearing assembly is mounted in the coating chamber through an aperture in the chamber sidewall, leaving only portions of the drive shaft, and the water and electrical conduits outside the chamber. This results in space being occupied by the bearing assembly which could otherwise be occupied by a longer cathode.
The cathode is permanently attached to the bearing assembly by welding the cathode body to an end plug which is an integral part of the bearing assembly. When such a cathode is removed from the coating chamber, for example to replace an eroded target, the entire bearing assembly including the cathode must be removed. Such an assembly may be removed through the top cover of the coating chamber, which means that a clearance space must exist between the free end of the cathode and the adjacent chamber end wall. The clearance space is necessary to allow the bearing assembly, and the drive and fluid connections to be moved free of the mounting aperture in the chamber side wall so that the assembly can be lifted out of the chamber. The clearance space also represents space which could otherwise be occupied by a longer cathode.
The weight of the bearing assembly is usually such that an overhead crane is needed to lift the assembly from the coating chamber. The use of a crane increases the time required for cathode repair and replacement. An in-line coating system may include as many as fifteen magnetrons, more than one of which may have to be removed at any given time. Usually, only one overhead crane is available. Therefore, cathode removal and replacement operations must be carried out in series. As such, the time required for these operations is increased, resulting in lost production time.
In view of the foregoing, an object of the present invention is to provide a cantilever mounted rotating magnetron incorporating a removable cathode so the cathode can be removed from the coating chamber without removing the bearing assembly.
It is a further object of the present invention to provide a cantilever mounted rotating magnetron wherein most of the bearing assembly is located outside the coating chamber, thus permitting the use of longer cathodes.
It is yet another object of the present invention to provide a cathode that may quickly and easily removed from the bearing assembly.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. Other objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the claims.